The overall objective of this project is to prepare a flexible and comprehensive integrated system of computer programs for the determination of macromolecular structures by X-ray crystallography. The system will carry out calculations involved in data acquisition, structure solution, model building, and structure refinement. We are developing a new technique for refining atomic coordinates of macromolecules. This techniques is based on a constrained least-squares treatment of atomic shift vectors obtained from difference electron density syntheses. Programs are being written for phase refinement and structural solution by molecular replacement methods. New formulations are proposed for absorption corrections and local scale factors in terms of Fourier-Legendre series. Parallel versions of the system are being written for use on minicomputers and large-scale computers. Thorough testing and detailed documentation are being carried out as integral steps in the preparation of the system. Where appropriate, testing is being carried out with actual experimental data. The system will be developed as a kind of research resource and made freely available to all interested crystallographers.